Mastering Ninja and then starting Samurai with the last ability you get from the Ninja (I forget the name, allows you to use 2 weapons) and put it on the Samurai is good. I mastered Blue Mage before the end of (SPOILER)World One and now I have my Faris as a permanent Summoner which is good as she can now concentrate on mastering that.

Although as Crono_Logical mentioned the X-Magic, I find a mixture of a Level 5 White Mage and a Level 5 Black Mage much more useful where I currently am (Forest of Mau, yes I am super weak, but the super in super weak makes me special).